1. Field of the Invention
The present invention relates to a laminate and a method for producing the same, and a functional element using the same. More specifically, the present invention concerns a laminate and a method for producing such a laminate, the laminate being configured in such a manner that a buffer layer, which is composed of an oxide thin film made of such as gallium oxide or an oxynitride thin film made of such as gallium oxynitride, is once formed on a substrate, and on the buffer layer, a transparent conductive film, which is mainly composed of titanium oxide and containing niobium or the like, and also made of a crystalline material containing an anatase phase, and further has high refractive index and low specific resistivity, is formed. The present invention also relates to a functional element provided with the above laminate, and in particular to a semiconductor light emitting element and a solar cell.
2. Description of the Related Art
The transparent conductive film, which has a high conductivity and a high transmittance in a visible light range, is desirably used for a flat panel display, a solar cell, an electrode for various other light receiving elements, etc., and is also utilized as a heat ray reflective film for automobile windows and construction materials, an antistatic film and a transparent heat generating body for use in various kinds of anti-fogging purposes, such as freezer showcases, etc.
As the practical transparent conductive film, tin oxide (SnO2)-based, zinc oxide (ZnO)-based and indium oxide (In2O3)-based thin films are proposed. Examples of the tin oxide-based thin films include those containing antimony as a dopant (ATO films) and those containing fluorine as a dopant (FTO films), and examples of the zinc oxide-based thin films include those containing aluminum as a dopant (AZO films) and those containing gallium as a dopant (GZO films), as well-known films. Now, indium oxide-based films have been most widely used as transparent conductive films in the industrial field. Among these, an indium oxide film containing tin as a dopant is referred to as an ITO (Indium-Tin-Oxide) film, and since the film makes it possible to easily provide a film having low resistivity, it has been widely utilized.
Moreover, most of transparent conductive films are made of semiconductors in which the n-type is degenerated. Therefore, electrons form the carrier, and the carrier concentration and mobility control the electric conductivity. In developments of the conventional transparent conductive film, attempts have been made to increase the carrier concentration in order to provide an ITO film having a lower resistivity.
The crystalline ITO film is made of an indium oxide phase in which tin is fused and solidified. This indium oxide phase forms a crystal structure referred to as “bixbyte” having a stable crystal phase of a cubic system under a normal pressure or a pressure lower than this. Therefore, the crystalline ITO film is allowed to generate a large number of carriers by oxygen deficiency as well as by replacing lattice points of trivalent indium in the bixbyte structure with tetravalent tin. Tin is an element that can increase the carrier concentration to the highest as a dopant, and it has been known that by adding 10% by weight of tin in tin oxide conversion, the lowest resistivity is achieved. The crystalline ITO film having its carrier concentration increased in this manner has remarkably superior characteristics, and has been used in various applications.
In recent years, there has been a fear of exhaustion of indium metal in the world, and its price has abruptly risen; consequently, developments of a transparent conductive film without using indium that can replace ITO films have been expected. Moreover, with current developments of various electronic devices, there have been strong demands for transparent conductive films that have a higher refractive index than the ITO film and exert a low electric resistivity equivalent to the ITO film.
As the application of such a transparent conductive film, typical examples thereof include a blue color LED and a solar cell. A gallium nitride layer is used as a light-emitting layer for the blue color LED, and one of optical characteristics of the gallium nitride layer is a high refractive index of about 2.4. On the other hand, in order to improve an extraction efficiency of light from the light-emitting layer, it is necessary to improve the matching characteristic of refractive indexes of the transparent conductive film and gallium nitride layer, and a refractive index close to 2.4 is required for the transparent conductive film.
The refractive index is a value inherent to each substance, and the refractive index of indium oxide generally known is as low as 1.9 to 2.0. Moreover, in the case when a gallium nitride layer is used for a light-emitting layer, a low surface resistivity is required for the transparent conductive film. The reason for this is because the gallium nitride layer has an electrical characteristic in that the current diffusion in a film surface direction is not sufficient. Here, in an attempt to lower the electric resistivity by increasing the carrier concentration of an indium oxide-based transparent conductive film, the refractive index is further lowered to 1.9 or less. In this manner, since the ITO film is a material whose carrier concentration is extremely increased by tin serving as a dopant, a problem arises in which in an attempt to obtain a crystal film with a low resistivity, its refractive index is lowered.
As the application in which characteristics superior to those of an ITO film are required for the transparent conductive film, a solar cell is listed as another example. Although the aforementioned FTO film is widely used for a surface electrode for the solar cell, solar light can be more effectively extracted by using a transparent conductive film having a higher refractive index. When used as one portion of the rear surface electrode, a transparent conductive film having an increased refractive index is sometimes used so as to improve the extraction efficiency for sun light and consequently to adjust the refractive index of the entire module, and in this case also, from the same reason as that for the blue color LED application, a transparent conductive film having a high refractive index is effectively used because the FTO film or the ITO film fails to provide a sufficient effect.
As the transparent conductive film having a refractive index higher than that of the ITO film, a transparent conductive film, which is prepared by adding a pentavalent or more element to titanium oxide having a refractive index higher than that of indium oxide, has been examined.
For example, Japanese Patent Application National Republication (Laid-Open) No. 2006-016608 has proposed a transparent metal material and a transparent electrode, which are formed by a material that is transparent and conducive, can be supplied stably, and is superior in chemical resistance. In this case, the description has proposed that by forming a metal oxide layer made of an anatase-type crystal structure, with the metal oxide layer being composed of M:TiO2, a low resistivity is exerted while properly maintaining an inner transmittance, and that by forming M:TiO2 as a result of substitution of Ti sites of the anatase-type TiO2 with another atom (such as Nb, Ta, Mo, As, Sb, or W), the electric conductivity can be greatly improved, with transparency being properly maintained.
Moreover, Japanese Patent Application National Republication (Laid-Open) No. 2006-016608 has also proposed that a titanium oxide-based transparent conductive film is used for a transparent electrode for a blue LED. In particular, since the refractive index of an anatase-type TiO2 is about 2.4, this film is optimally used in an attempt to provide well-matched characteristics between refractive indexes of the blue LED and the gallium nitride layer. Japanese Patent Application National Republication (Laid-Open) No. 2006-016608 has described that after an alignment film, such as a ZnO film, a ZrO2 film, an SrTiO3 film, an MgO film, an LaAlO3 film, a CeO2 film, a ZrO2 film or an Al2O3 film, has been preliminarily formed on a substrate, a TiO2 film is formed thereon so that the alignment film functions as a buffer film (buffer layer). However, with respect to this description, since the expression “considered” is used, and since no examples are given to the alignment film, it doesn't leave the level of the supposition.
Japanese Patent Application Laid-Open No. 2008-50677 has proposed a metal oxide film made of a titanium dioxide film having an anatase structure in which one or two or more dopants selected from the group consisting of Sn, Hf, Si, Zr, Pb and Ge, and one or two or more dopants selected from the group consisting of Nb, Ta, Mo, As, Sb and W, are added to the titanium dioxide film, and the literature describes that by using this structure, while controlling a refractive index and a wavelength band forming a transmittance range by the concentration of the one or two or more dopants selected from the group consisting of Sn, Hf, Si, Zr, Pb and Ge, its resistivity is reduced by using the one or two or more dopants selected from the group consisting of Nb, Ta, Mo, As, Sb and W so that a metal oxide film having superior optical characteristics with a low resistivity can be obtained.
Moreover, Japanese Patent Application Laid-Open No. 2008-84824 has proposed a method for manufacturing a conductor characterized by including a step of forming on a substrate a precursor layer made of titanium oxide to which one or two or more dopants selected from the group consisting of Nb, Ta, Mo, As, Sb, Al, Hf, Si, Ge, Zr, W, Co, Fe, Cr, Sn, Ni, V, Mn, Tc, Re, P and Bi are added, and a step of forming a metal oxide layer by annealing the precursor layer under a reducing atmosphere, and the literature has described that by using this method, a conductor that has a good conductivity and is also superior in transparency can be obtained.
Furthermore, Japanese Patent Application Laid-Open No. 2009-231213 has proposed a conductor having a good conductivity and is superior in heat resistance and a method for manufacturing such a conductor, and described, in particular, that the conductor is characterized by a structure in which two or more layers, made of titanium oxide to which one or two or more dopants selected from the group consisting of Nb, Ta, Mo, As, Sb, W, N, F, S, Se, Te, Cr, Ni, Tc, Re, P and Bi are added, are formed on a substrate, with at least one layer of the two or more layers being a second layer in which a ratio of the number of atoms of the dopants relative to the total number of atoms of titanium and the dopants is set to 0.01 to 4% by atoms, and with a first layer having a more ratio of the number of atoms of dopants relative to the total number of atoms of titanium and the dopants than that of the second layer being formed between the second layer and the substrate.
However, with respect to Japanese Patent Application National Republication (Laid-Open) No. 2006-016608, Japanese Patent Application Laid-Open No. 2008-50677, Japanese Patent Application Laid-Open No. 2008-84824 and Japanese Patent Application Laid-Open No. 2009-231213, it has been known in general that it is difficult to directly form a titanium oxide thin film of an anatase structure onto a substrate. For example, it has been described that the direct formation is available only in the case of using a limited substrate, such as SrTiO3 or the like as described in Japanese Patent Application National Republication (Laid-Open) No. 2006-016608.
On the other hand, by utilizing a high refractive index of a titanium oxide-based transparent conductive film, the use thereof as a functional element, in particular, as a transparent electrode of a semiconductor light emitting element, has been examined.
For example, Japanese Patent Application National Republication (Laid-Open) No. 2006-073189 has proposed a functional element characterized by including AlxGayInzN (0≦x≦1, 0≦y≦1, 0≦z≦1), known as a light-emitting layer for blue LED, and an oxide material made of metal oxide formed on the AlxGayInzN, with the metal oxide being prepared as TiO2 or the like, and described that the resulting functional element has a structure in which a film that is less reflective on the interface, and compatibly has a good chemical resistance and durability, is integrally formed on a III group nitride having superior physical and chemical characteristics.
Moreover, Japanese Patent Application Laid-Open No. 2008-294306 has proposed a III group nitride-based compound semiconductor light emitting element in which a sapphire substrate, a buffer layer made of aluminum nitride (AlN), an n contact layer, an n clad layer, a multiple quantum well layer having a light emitting wavelength of 470 nm, a p clad layer and a p contact layer are formed, and a light transmitting electrode having irregularities made of titanium niobium oxide is formed on the n contact layer, with an electrode pad being formed on one portion of the electrode. The Japanese Patent Application Laid-Open No. 2008-294306 has described that, since the light transmitting electrode is formed by adding 3% of niobium to titanium oxide, the refractive index thereof to wavelength 470 nm is made virtually the same as that of the p contact layer, so that the total reflection on the interface between the p contact layer and the light transmitting electrode can be substantially avoided, with the light-extraction rate being improved by 30% by the irregularities.
However, although there is a description that a titanium oxide thin film to which niobium is added is epitaxially grown on gallium nitride in Japanese Patent Application National Republication (Laid-Open) No. 2006-073189 or Japanese Patent Application Laid-Open No. 2008-294306, there is no description indicating the fact that a titanium oxide thin film of an anatase structure is directly formed thereon. In contrast, as described in “Heteroepitaxial Growth of Rutile TiO2 on GaN (0001) by Pulsed Laser Deposition”, Japanese Journal of Applied Physics, 2005, Vol. 44, p. L1503-L1505 by the inventors of Japanese Patent Application Laid-Open No. 2009-231213, the fact that on the gallium nitride, a titanium oxide thin film with niobium added thereto having not an anatase structure, but a rutile structure having a low conductivity is epitaxially grown has been reported.
In other words, to form a titanium oxide thin film with niobium added thereto having an anatase structure with a high conductivity on gallium nitride by an epitaxial growth or the like is still left as a subject to be achieved. Moreover, there have been strong demands for forming a titanium oxide thin film with niobium or the like added thereto having an anatase structure with a high conductivity on the outermost surface layer of a substrate, regardless of the crystal structure of the outermost surface layer of a functional element forming the substrate.